The invention concerns a process and an apparatus for producing hollow bodies of plastic material, and more particularly a process and an apparatus for producing hollow bodies of thermoplastic material which are at least partially filled with foam.
Particularly in relation to the manufacture of components for the automobile industry, it is known for hollow bodies of plastic material to be produced for example by a blow molding procedure, whereupon the hollow bodies in the expanded condition are subsequently filled with plastic beads of foam material, also referred to as mini-beads, in order in that way to form a foam filling within the hollow body. In this respect attention may be directed by way of example to German laid-open application (DE-OS) No 196 48 525.
The processes used hitherto for producing automobile components of that kind are fairly complicated as the procedure involves firstly producing the actual hollow body itself, in order subsequently to fill it with the plastic beads in a separate working operation. Amongst other things that is a time-consuming operation. Added to this is the fact that the multiple handling procedures, for example when removing the hollow body from the mold, transporting it to an operating station in which the plastic beads are introduced into the hollow body, when fitting the hollow body into a support mold which is necessary for that phase, and so forth, can mean that the surface of the hollow body frequently suffers damage which, even if it only entails scratches or the like, can adversely affect the appearance of the finished component or make it more difficult to apply a coating of lacquer or paint, or the like.
An object of the present invention is so to modify a process for producing hollow bodies of thermoplastic material which are at least partially filled with foam, that, while nonetheless affording reliable results, the production of such plastic bead-filled components can be simplified, in particular also in the sense that the hollow body for manufacture of the final product has to be transported or otherwise handled to a reasonably minimum degree.
Another object of the present invention is to provide a process for producing a hollow body from thermoplastic material with an at least partial foam filling, which entails a more rational operating procedure with operating steps in a more logical sequence aimed at minimising potentially quality-hazardous transfer procedures.
Still another object of the present invention is to provide an apparatus for producing hollow bodies of plastic material which are at least partially filled with foam, which can provide for minimising the number of operating components involved in terms of transportation of articles from one part of the apparatus to another.
In accordance with the principles of the present invention the foregoing and other objects are attained by a process for producing hollow bodies of plastic material, which are at least partially filled with foam, wherein firstly a preform is expanded in a blow molding mold by an increased internal pressure therein to form a hollow body therefrom. The hollow body is then at least partially filled with plastic beads of foamed plastic and the plastic beads experience an increase in volume within the hollow body. In this procedure the plastic beads in a compressed condition are introduced into the expanded hollow body while still within the mold, under a pressure greater than the pressure obtaining in the hollow body. The plastic beads in the hollow body then experience a relief of pressure which results in an increase in the volume of the plastic beads within the hollow body, which provides for as complete filling as possible with a plastic foam of the at least one delimited part of the volume of the hollow body which is to be filled with foam.
In the apparatus aspect of the invention the foregoing and other objects are attained by an apparatus for producing hollow bodies of thermoplastic material which are at least partially filled with foam, the apparatus including at least one blow molding mold comprising first and second mold portions, and a device for producing in a preform disposed within the mold cavity of the mold an internal pressure which is higher than the external pressure surrounding the preform, thereby to expand the preform to afford a hollow body. At least a delimited part of the volume of the interior of the hollow body while still in the mold can be communicated with a container for plastic beads which in operation are contained therein under an increased pressure, by way of a closable conduit forming a communication between the mold and the container.
As will be seen in greater detail from an embodiment of the process and apparatus of the invention as set forth hereinafter, the process of the invention provides that firstly a blow molding procedure is used to produce from a preform of suitable material a hollow body which determines the external configuration of the component to be manufactured, for example a bumper for a motor vehicle. After the expansion operation, possibly already beginning in the last phase thereof, a plurality of plastic beads of foam material are injected into the body while still in the mold. The beginning of that injection procedure depends substantially on the pressure conditions obtaining in the hollow body. In general it will be desirable firstly to reduce the preform-expansion or blowing pressure which obtains within the hollow body which has been expanded from the preform, and which can be for example between 10 and 12 bars, so that the plastic beads can be injected into the interior of the hollow body under a comparatively low operating pressure, for example between 2 and 4 bars.
Those small plastic beads which preferably comprise closed-cell plastic material, are therefore put under an increased pressure so that they have thereby experienced a noticeable reduction in volume and are in accordingly a compressed condition. The hollow body while still in the mold, or at least the part of the hollow body which is to be filled with foam, is filled as completely as possible with the plastic beads so that, after the internal space enclosed by the hollow body has been completely relieved of the internal pressure therein, the plastic beads experience a relief of pressure and thus are involved in a certain expansion effect and increase in volume, which generally provides that even regions which have not yet been filled up in the interior of the hollow body are filled by the foam material of the plastic beads. The increase in the volume of the plastic beads can mean that the hollow body is permanently under a certain increased internal pressure. That however does not represent a problem. On the contrary, it has been found that in the above-described manner it is possible to arrive at a closed uniform foam filling within the hollow body, without the necessity for the plastic beads to be joined together by means of welding or in some other fashion. It will be noted however that, with the process according to the invention, it is possible for the plastic beads which bear against the wall of the hollow body to be welded thereto. That will be the case when using compatible materials, whenever the wall of the hollow body is still in a soft plastic condition at the beginning of the operation of injecting the plastic beads which will therefore adhere to the wall of the hollow body.
It is possible to use practically any suitable plastic material which is foamable, for example also polyurethane, for the plastic beads. For the purposes of simplifying disposal however it is desirable for the plastic beads to be made from a material which also constitutes the hollow body. In the case of vehicle bumpers, polypropylene is frequently used for the hollow body. Accordingly in that case the plastic beads should desirably also be made from polypropylene. It is however also possible to use other plastic materials, in particular thermoplastic materials, for production of the hollow body and the plastic beads.
It will be noted that in accordance with a further feature of the invention it is also possible to proceed in such a fashion that the hollow body which for example represents the housing of a vehicle bumper is subdivided into a plurality of chambers which are delimited from each other and which can possibly be filled independently of each other with the plastic beads. In that respect there is the possibility of for example plastic beads of different sizes or plastic beads of properties which differ in other respects being introduced into the individual chambers which are closed off relative to each other, thus affording for example a vehicle bumper which enjoys different properties in terms of absorbing shocks and impacts thereagainst, its flexibility and so forth, in certain regions thereof.